1. Field of the Invention
The present invention relates to a novel layered composite oxide and, in further detail, relates to a layered composite oxide having a B-site controlled, layered perovskite-type structure and having excellent oxygen ion conductivity. The layered composite oxide of the present invention is advantageous for various catalysts and is particularly preferably used for a diesel particulate filter (DPF) configured to purify a particulate matter (PM).
2. Background Information
Materials having perovskite-type structures are represented by the general formula ABO3 and ideally belong to the cubic system but frequently have distorted structures belonging to the tetragonal system, the orthorhombic system, the six-law system, and the like. The materials having such perovskite-type structures have attracted attention as electromagnetic materials and catalyst materials due to the distortion.
In recent years, attempts have been made to synthesize layered perovskite composite oxides exhibiting excellent characteristics by layering various materials having perovskite-type structures. For example, some of the materials attract attention as materials for carbon monoxide (CO) shift catalysts and electrolyte membranes of fuel cells (FC). Also, for exhaust gas purification catalysts, there are found reports on decomposition of nitrogen oxides (NOx), but high-temperature regions apart from a practical temperature region are still employed.
On the other hand, materials having a LaGaO3-based perovskite-type structure represented by the general formula ABO3 have been known as catalysts for PM purification in internal combustion engines, and it has been reported that PM oxidation temperatures of PM oxidation catalysts containing platinum supported on the materials are preferably 450° C. (refer to, for example, Japanese Unexamined Patent Application Publication No. 2006-341265).